Turbine engines are used as the primary power source for various kinds of aircraft and other vehicles. The engines may also serve as auxiliary power sources that drive air compressors, hydraulic pumps, and industrial electrical power generators. Most turbine engines generally follow the same basic power generation procedure. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge onto turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more air into the engine, and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, electrical generators, or other devices.
Both airfoils and combustors made from silicon nitride or silicon carbide have the potential to appreciably increase the operating temperatures of turbine engines. The high temperature and high pressure environment of the turbine engine as well as the high gas velocity can cause erosion of silicon based ceramics. The mechanism of some of the erosion loss is due to the formation of SiO2 and SiO gas. Typically, combustion gas environments, including turbine engines, contain about 10% water vapor. Oxygen containing water in the turbine reacts with silicon nitride and silicon carbide to form silica scale on silicon based ceramic surfaces. Water vapor can also react with the silica scale to form silicon hydroxide, which is volatile. Evaporation of silicon hydroxide from ceramic surfaces and erosion of ceramic caused by high speed combustion gases passing over ceramic surfaces leads to the loss of ceramic material from ceramic combustor and turbine components at rates of a few microns per hour.
U.S. Pat. No. 6,159,553 and US 2002/0136835 A1 show protective ceramic coatings. Tantalum oxide alloyed with lanthanum oxide provides an environmental coating (EBC). However, tantalum oxide permits diffusion of oxygen, resulting in the formation of a SiO2 layer below the tantalum oxide layer. Published U.S. patent application 2002/0098391 by Tanaka et al discloses the use of rare earth silicates to form a protective coating to a silicon based substrate ceramic material. But the process disclosed by Tanaka limits the coating composition because it allows interaction of the coating with the substrate.
Accordingly, there is a need for an improved coating and method to apply the coating for a high temperature (>2200° F. (>1200° C.)) barrier between an environmental coating and a substrate of silicon nitride or silicon carbide. There is also a need for a diffusion coating that will prevent migration of cations out of a silicon-based substrate. There is as well a need to coat complex parts with a uniform dense oxidation resistant coating at a minimal cost. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.